Absolute total positive-ion electron ionization cross-sections from threshold to 220 eV are reported for a range of halogenated methanes and small perÑuorocarbons (2È4 carbon atoms). Correlations between the measured ionization cross-section and related molecular properties, in particular the vertical ionization potential (or vertical appearance energy) and molecular polarizability volume, are noted. Contributions to the total cross-section from individual bonds are also determined. Cross-sections predicted using these " bond contributions Ï are in agreement with experiment for a wide range of molecules to better than ^10% accuracy, and in most cases to better than ^5%. The experimental data are also compared with ionization efficiency curves calculated using the (DM) and binary encounter Bethe (BEB) models.

Absolute total electron-impact ionization cross sections from threshold to 220 eV are reported for the formation of positive ions from a range of chlorocarbons (one to five carbon atoms), including all chlorine-substituted methanes and ethanes. Correlations between the measured ionization cross section, ionization potential and molecular polarizability volume are explored and compared with data for the perfluorocarbons and mixed halocarbons. A C-Cl bond additivity cross section determined previously for mixed halomethanes has been refined to fit the experimental data for the higher chlorocarbons. Maximum cross sections predicted using bond additivity contributions are shown to be in agreement with experiment for a wide range of molecules to better than ±10% accuracy, and in most cases to better than ±5%. The experimental data are compared with the predictions of the Deutsch-Märk and binary-encounter Bethe models.

Reprinted with permission from the American Physical Society: Phys. Rev. A 88, 062710 c (2013) by the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modied, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society.Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Perfluorocarbons (PFCs) are man-made compounds whose ion physics exhibit complex interplays between statistical and nonstatistical fragmentation and intramolecular rearrangement processes. One probe of such processes is the energy-dependent electron-impact-ionization cross section. Partial electron-impact-ionization cross sections are reported for the fragments arising from five C 2 to C 4 PFCs, namely, C 2 F 6 , C 3 F 8 , C 3 F 6 , CF 2 =CF-CF=CF 2 , and CF 3 -C≡C-CF 3 , over the energy range from threshold to ∼210 eV. Care was taken to maximize ion collection efficiency and to minimize discrimination against ions produced with high kinetic-energy release, and the measured cross sections have been calibrated using independent absolute total (gross) ionization efficiency curves measured previously in the same laboratory with an instrument that was designed to essentially have unit detection efficiency. Total ionization cross sections have also been modeled using the binary-encounter Bethe model, and the shortcomings of the model when applied to perfluorinated compounds are discussed. Analysis of the mass spectral fragmentation patterns in combination with ab initio energetics suggests that nonstatistical dissociative ionization processes play a significant role in the fragmentation dynamics of saturated PFCs. In contrast, unsaturated PFCs exhibit long-lived parent ions, which tend to undergo a higher degree of statistical dissociation following ionization, involving considerable intramolecular rearrangement.

In this article we introduce the application of ion imaging to the detection of product ions produced in electron–molecule collisions. Unlike photoinitiation experiments in which a short duration laser pulse is used to excite a reagent molecule or to prepare an exotic species for subsequent reaction, collisions involving electrons (or ions) involve field-sensitive charged beam pulses of longer duration than laser pulses. Such experiments require the fast switching of electric fields with particular attention to the elimination of stray fields, contact potentials, and noise pickup on cables and electrical feedthroughs. We have recorded preliminary O2+ and O+ ion images from 100 eV electron impact ionization of O2 supersonic molecular beams. The triggering of the nozzle, field plates, electron gun, and repeller/extractor elements was achieved using a low cost, versatile, fast multichannel pulse generator with fiber-optic coupling that was designed to produce low noise trigger pulses for commercial and custom made pulsed high voltage power supplies.

No abstract